1. Field of the Invention
This invention relates to magnetomechanical transducers, that is transducers which utilize magnetomechanical resonant frequency shifts, and more particularly to transducers capable of measuring pressure via displacements and/or rotations of pressure sensitive devices.
2. Description of the Prior Art
Transducers of the type upon which this invention has improved are conventionally used as measurement devices to obtain an electrical signal representative of some physical quantity: for example displacement, speed, pressure and the like. Several of these transducers are presently used to measure displacement and angles.
One type of transducer applies the properties of variable inductance in measuring movement. Variable inductance transducers consist of one or two coils, a core, an armature, an exciter and an induction meter. When a simple single coil is used as a transducer element the mechanical displacement of the armature changes the permeance of the flux path generated by the coil, thereby changing the inductance of the coil. The change in inductance is then measured by suitable circuitry and displayed in appropriate units. The main disadvantage of this transducer is mechanical loading (altered displacements and slower dynamic response) of the signal source resulting from the attachment of the armature mass to the signal source.
Capacitive transducers provide a second method of measuring displacement. Capacitive transducers operate in two modes, changing dielectric constant or changing area. The changing area mode is used in displacement measurements. The changing area is created by shifting capacitor plates in such a way that the effective surface areas of the opposing plates are varied. This creates a change in capacitance which can be measured and correlated to desired units of displacement. The changing dielectric constant is most commonly used in measuring fluid levels. By replacing the vapor between the plates of a capacitor with liquid, the effective dielectric constant can be altered and thus the capacitance will vary with respect to the liquid level. The disadvantages of capacitance systems occur in the area of signal source loading. Capacitors require a closed active circuit to operate, thus the capacitor plate attached to the signal source must have an electrical connection to close the circuit which restricts the motion of the freely moving object in addition to increasing the objects dynamic response time.
Another type of transducer often referred to as a linear variable differential transformer or LVDT provides an ac voltage output proportional to the relative displacement of a transformer core to its windings. The LVDT is a mutual inductance device using three colinear coils and a linearly movable core. The center coil is energized from an external ac power source and the two end coils, connected together in phase, are used as pick-up coils. Output amplitude and phase depend on the relative coupling of the two pick-up coils to the power coil. Relative coupling is in turn dependent on the position of the core. There is a core position for which voltage induced in each of the pick-up coils will be of the same magnitude but opposite in phase, resulting in zero net output. The output voltage magnitudes are ideally the same for equal core displacements on either side of null balance. However, the phase relation existing between the power source and output changes 180 degrees through null. It is therefore possible through phase determination or the use of phase sensitive circuitry to distinguish between outputs resulting from displacements on the two sides of the null. The LVDT offers several distinct advantages over many alternative transducers. First serving as a primary detector-transducer it converts mechanical displacement into a proportional electrical voltage which is a desirable output. The LVDT cannot be overloaded mechanically since the core is completely separable from the rest of the device. It provides a comparatively high output and is reusable, making it one of the most widely used transducers.
The disadvantages of the LVDT are similar to those of the previously described transducers. One disadvantage is that intermediate amplification is required if the output signal is to be transported over a relatively short distance (e.g. more than 5 ft.). The LVDT is limited to relatively linear displacement measurements and short displacement detection range verses overall device length. The LVDT and corresponding measuring devices are expensive to manufacture, thus giving the LVDT transducer system a fairly high cost. In addition dynamic measurements are hindered because the core is of appreciable mass. The exciting frequency of the carrier may also be a limiting factor. The axial force exerted by the core on the signal source is larger when the readily available 60 Hz power source is used than when power source with higher frequencies are used. In addition, the advantage of simple circuitry is lost if the direction from the null must be indicated.
One of the major problems with these transducers is the difficulty of obtaining an accurate measurement without altering or disturbing the system to be measured. These transducers are inefficient in that they require intermediate amplification if a remote reading is required, which results from their sensitivity to amplitude variations in their signals. Another problem with such transducers is the relatively high cost thereof.